Conventionally, a denitration apparatus decomposing nitrogen oxides contained in a combustion gas discharged from a combustion engine such as a gas turbine, and preventing an adverse effect on an atmospheric environment has been known. Further, it is known that when the combustion gas with temperature exceeding allowable temperature flows into the denitration apparatus including a catalyst part which decomposes nitrogen oxides, performance degradation of the denitration apparatus, or a failure of the denitration apparatus occurs. There is known a denitration apparatus in which a cooling apparatus which cools a combustion gas is installed at an upstream side of a catalyst part in order to prevent the troubles like this (refer to, for example, Patent Literature 1). As methods for cooling a combustion gas, a method for transferring thermal energy of a combustion gas to a cooling medium and a method for mixing a cooling medium and a combustion gas are known.